The present invention relates to breast imaging devices and methods using non-ionizing radiation of narrow spectral bandwidth, particularly, enhancing the images obtained by such devices and methods.
X-ray mammography based on film-screen or xeroradiographic detection has for years been commonly accepted as a mass screening technique for breast disease. However, certain risks are associated with x-ray examination since x-ray radiation is also ionizing. Because exposure to such ionizing radiation should be minimized, the frequency and number of exams should be limited. Therefore, when using x-ray examination it is important to screen a patient""s breast properly on the first attempt.
In recent years, broad beam light sources (sometimes referred to as xe2x80x9clight torchesxe2x80x9d) having a wide spectral bandwidth in the visible and infrared ranges have been used for breast imaging. Broad beam light transmitted through a breast is typically recorded by a video camera, converted to an analog signal and viewed on a video monitor, or is digitized and analyzed on a computer. However, the ability to discriminate between various tissue-types in a breast via this technique is reduced if the transmitted beam has a wide spectral bandwidth (i.e. contrast is lost). Light may be absorbed, transmitted, scattered, and/or reflected to different degrees by different tissue types making it difficult to obtain information about the nature of any of the tissue. In addition, resolution and contrast may be lost due to a large amount of scattered light being transmitted from the breast being imaged to the detector. Resolution of images resulting from broad beam light source imaging is far below that which can be obtained with x-ray imaging systems. Detection limits when using this technique have generally been of lesions no smaller than what a physician can detect by palpitation. Therefore, this technique is not particularly advantageous.
As the present applicants described in now issued U.S. Pat. Nos. 4,649,275, 4,767,928, 4,829,184, and 4,948,974, a collimated (i.e. focused) light (i.e. non-ionizing radiation) source of narrow spectral bandwidth (such as is generated by a laser, a waveguide, a phased array, etc.) can be used to produce a beam or a number of beams of small spatial dimensions appropriate for acquiring images of a breast with high spatial contrast resolution. The narrow spectral bandwidth improves the characterization of the composition of the breast material being imaged to be more detailed. More information can be obtained by acquiring additional images at other wavelengths with narrow spectral bandwidths.
For example, FIGS. 1 and 2 depict an apparatus for mammographic (breast imaging) applications which entail using collimated light (i.e. non-ionizing radiation in the near ultraviolet, visible, infrared, microwave, etc.) of narrow spectral bandwidth to obtain high resolution images. Appropriate sources of light in the visible and near-infrared spectrum include lasers or filtered light sources. As is shown in FIGS. 2a-2d, it is preferred that the light source be positioned on one side of the breast to be imaged and a receiver, such as a photodetector, be positioned on the opposite side to record transmitted light. As is shown in FIGS. 1 and 2, it is preferred that the breast be compressed between compression plates. The amplitude of a light beam, as well as other possible properties such as beam coherence, polarization, angular and spectral distribution, will be altered by absorption, reflection and refraction as it propagates through the breast and plates. Image resolution can be controlled by adjusting the cross-sectional area of light beam(s) before and/or after transmission through the breast.
The electromagnetic properties of various normal and diseased breast materials may exhibit wavelength dependence. Thus, acquiring images at different wavelengths of light may aid in distinguishing tissue types and calcifications. As can be appreciated from FIG. 1b, light beams of wavelengths xcex1 and xcex2 sent from sources 1 and 2 are incident normal or nearly normal to the surface of one compression plate. The transmitted light is attenuated by the two plates and the breast material and then detected. An image or images can be acquired by simultaneously translating one or more light source and detector combination past the breast. As is shown in FIG. 1b, it is preferable that each light source emits a different wavelength of light (i.e. xcex1xe2x89xa0xcex2). If a single light source provides more than one distinct wavelength, then a means of separating the wavelengths (narrow spectral bandwidth filters such as absorptive glass, transmissive or reflective gratings, etc.) is preferably incorporated prior to light reaching the detector or a detector which is sensitive to only a subset of the wavelengths employed is preferably used.
High resolution images may be obtained with a variety of scanning techniques: FIGS. 2a and 2b show a point beam or multiple point beam which could be used in a raster scan format. The transmitted light beam can be collimated by a simple air gap, fiber optics, amplified fiber optics, light-pipes, focused lenses, waveguides, focused arrays, masks, polarization filters, narrow spectral bandwidth filters (which can also be directionally sensitive), or mechanical apertures to minimize detection of scattered light. This approach can be extended to include a single line or multiple line scan format as shown in FIG. 2c. High speed two-dimensional imaging is shown in FIG. 2d. In this case collimation (such as fiber-optics or light pipes) can be introduced into one or both compression plates. In all cases collimation may be used to produce a beam or beams of small cross-section and directional nature. These attributes can be used to exclude transmitted scatter from the exit beam.
If two or more sources providing light beams of differing wavelengths are spatially separated as shown in FIG. 1b, then narrow spectral bandwidth filters can be used between plate B and the detectors for each wavelength such that the detector for xcex2 rejects light of wavelength xcex1 which is scattered into the path of the xcex2 beam. In this case the spectral filter functions as a collimator, rejecting a component of the transmitted beam which can only be attributed to scatter. This would be advantageous when continuous sources are utilized or sources are pulsed almost simultaneously. The ability to reject wavelengths outside of a narrow spectral bandwidth would allow the source for xcex1 to be located closer to the source for xcex2, improving image acquisition speed. Of course, a narrow bandwidth spectral filter can also provide directional discrimination (for example, diffraction gratings, interferometers, etc.). Although using two sources with different wavelengths is described, two sources with different polarizations could be used.
By positioning source 1 (for xcex1) adjacent to source 2 (for xcex2) the scatter contribution from source 1 into itself (near the boundary with source 2) can be estimated by measuring the xcex2 component at the location of source 1. This assumes that radiation of wavelengths X, and xcex2 have similar scattering and absorption properties for the type of tissue being imaged. Another technique is to have sources 1 and 2 incident at the same location, but source 2 would be tilted with respect to source 1.
The light which is transmitted and recorded by the detector represents the attenuated beam plus scattered light. The light entering and exiting the breast can be collimated by introducing a patterned (structured) collimator (e.g. through the use of masks) so as to reject much of the scatter component. Collimation can be introduced before the photodetector to reduce the level of this scattered light. The photodetector produces an analog signal which can be displayed or digitized for storage and analysis on a computer.
As is disclosed in applicants"" prior patents, one instance of collimation of light entering the breast and of light exiting the breast (i.e. xe2x80x9cpost-collimationxe2x80x9d) to reduce the detected scattered light can be achieved through use of masks or virtual masks. A checkerboard pattern, as shown by way of example in FIG. 3, may be used. In one embodiment the checkerboard mask would be interposed between the source and the breast. Radiation from a source, as for example a line source or a two-dimensional source, is blocked partially by the opaque portions of the checkerboard mask prior to transmission through the breast. Use of the mask results in a reduction of detected scattered radiation since the multiple sources created by the mask are now spatially separated. The complete object to be imaged is scanned by moving the checkerboard mask by one square such that a region which had previously been covered by an opaque region is now covered by a transparent region and vice versa. Optionally, an identical mask aligned with the mask may be used after the breast to further limit detected scatter. Mask patterns other than a checkerboard may be used, for example, masks with hexagonally shaped transparent and opaque regions.
Virtual masks may also be employed. Such a virtual mask is shown by way of example in FIG. 4. Sources 10 are spaced apart so as to transmit radiation at locations less than covering the whole field (e.g. which are not continuous). The sources 10 may be light pipes, which are then spaced apart from each other. Other types of sources may be used. An image is acquired by moving the virtual mask to new locations until all points have been scanned. Imaging of the breast with optical methods may be accomplished by techniques described elsewhere. See, for instance, U.S. Pat. No. 4,515,165, issued May 7, 1985 to Carroll. Additional post collimation techniques (i.e. collimation of exiting radiation) for scatter reduction which can be used for pulsed or continuous radiation sources may include air gaps, fiber optics, amplified fiber optics, light pipes, mechanical apertures, polarization filters, focused lenses, focused arrays, waveguides, and wavelength-selective filters. The conventional masks (and virtual masks) described depend on the spatial separation of the sources. The ability to separate adjacent sources on the basis of radiation properties (wavelength, polarization, coherence, etc.) allows the superposition of multiple source-mask units, each with distinct radiation properties.
It will be appreciated that optical (non-ionizing radiation) tomography utilizing a collimator can be employed in a variety of fashions. As shown in FIG. 6, an object, such as a breast 30 may be imaged by a source of radiation 32 generating a one or two dimensional radiation beam, a detector 34, and a collimator 36 disposed between the source 32 and the detector 34. In this way multiple two dimensional images may be obtained simultaneously, thereby providing a three dimensional image of the object. For example, as shown in FIG. 7, a line source 42 or linear array of point sources may irradiate the object to be scanned such as a breast 44. Transmitted radiation then passes through a collimator 46, and then is detected by a detector 48, such as a two dimensional array of detectors, or a camera.
An optical structured (patterned) collimator such as a fiber optical bundle, mask or honeycomb-like device introduces its own transfer function into the transfer function of the imaging system (which includes the source and its collimator, the detector and its collimator, and the optical properties of the breast). Thus, the signal recorded by the detector(s) represents the superposition of all elements of the imaging chain. Many techniques have been developed in the field of image processing to attempt to correct for the effects of those elements of the imaging chain which have a non-negligible impact on image quality. In the case of an optical structured (patterned) collimator it may be preferable to reduce or minimize the contribution of its transfer function prior to the signal reaching the detector. For an optical structured post-collimator such as a fiber bundle, the fibers (elements) in the fiber bundle do not occupy 100% of the bundle cross section: there are xe2x80x98deadxe2x80x99 (non-imaging) regions between the individual fibers. In addition, a fiber of the fiber bundle may be seen by more than one detector element. The adverse effects of an optical structured post-collimator pattern (such as a fiber bundle) on image quality can be reduced by moving the optical structured (patterned) collimator(s) in a reciprocating fashion in front of the detector(s), thereby blurring the image of the collimator and improving overall definition and resolution of the desired breast image.
Also previously disclosed, a desirable imaging format is to have the collimated light (radiation) beam(s) incident normal to the surface of the breast and exit from the breast normal to the breast surface. However, breasts often have irregular shapes. To reduce any problems associated with light incident on and transmitted out of surfaces which are not necessarily normal to the direction of beam transmission, it is desirable to flatten the entrance and exit breast surfaces. This is easily accomplished using a pair of transparent, flat plates as is shown in FIG. 1a. The breast can be placed between two transparent plates and compressed so as to establish good surface contact while at the same time reduce the path length of the transmitted light beam(s) through the breast. The compression technique is commonly employed in x-ray mammography.
However, conventional x-ray mammography entails compression of the entire breast using flat compression plates (plates) of essentially the same size. Typical x-ray mammography compression plate sizes are slightly larger than their corresponding film cassette sizes. Representative dimensions for typical compression plates are (approximately) 24 cmxc3x9718 cm and 30 cmxc3x9724 cm. The use of smaller compression plates for smaller breasts appears to be motivated by cost savings (film and film processor chemicals) rather than a physical requirement to employ a smaller plate size. Compression plates improve x-ray mammography imaging by:
1. Reducing the thickness of breast tissue that x-rays must penetrate (and thus reducing x-ray beam hardening, absorption, and scattering).
2. Flattening the entrance and exit surfaces of the breast and thus creating a relatively uniform thickness of tissue (except for the region near the nipple and breast tissue that bulges outward between the plates and thus loses contact with the plates).
3. Immobilizing the breast during image acquisition in order to minimize image blur.
Flattened, parallel entrance and exit surfaces define a uniform thickness of tissue (uniform within the contrast resolution capabilities of the x-ray mammography imaging system) over the area in which tissue is in direct contact with the compression plates. This ensures that changes in image contrast are the result of differences in tissue composition only and not variations in tissue thickness or irregular entrance or exit surfaces. Unfortunately, tissue near the nipple and tissue that bulges outward due to compression present a variable (non-uniform) thickness to the x-ray imaging system. This typically results in the nipple and bulging tissue areas of the x-ray image being over-exposed relative to areas of the x-ray image in which tissue is in direct contact with the compression plates. The impact of skin irregularities (indentations, creases, folds, etc.) and skin surface roughness (skin is porous) on a x-ray mammography image of a compressed breast tends to be negligible (based on the typical range of angles for incident x-rays).
It is highly desirable to reduce the compressed breast thickness (which also reduces the variation in thickness for bulging tissue or the nipple region). Unfortunately vigorous compression of the entire breast can be painful. The design and operation of x-ray mammography equipment are based on a number of assumptions (range of breast sizes and compositions, image acquisition times, the use of x-ray grids or air gaps between the detector and the exit surface of the breast, etc.). The need to calibrate or tune the equipment has resulted in the development of a FDA-approved calibration phantom made from plastic (roughly 4 cm thick) which simulates a 5 cm thick compressed breast of known composition. For this reason, a number of researchers developing systems for optical imaging of the breast have built flat, parallel wall test phantoms (box-shaped containers) filled with tissue-simulating materials to a thickness of 5 cm. This experimental configuration is based on the assumption that an optical mammography system needs to duplicate the typical image acquisition format of a x-ray mammography system (image acquisition with the entire breast compressed). This approach is extremely problematic since optical scattering and absorption in breast tissue are much more severe than x-ray scattering and absorption at x-ray mammography energies. In addition, the expectation is that actual optical image acquisition times for a breast compressed to a thickness of 5 cm would be much too long to maintain compression of the entire breast. Problems associated with imaging the nipple region and bulging tissue would still be present since the tissue thickness is variable. An added complication is that the optical radiation is sensitive to the index of refraction at the surface (interface) and the angle of incidence (which can be altered by factors such as surface roughness and an irregular surface).
In addition, it has been disclosed that the manner in which radiation interacts with a medium can be altered by the presence of an acoustic field. See, e.g., A. Korpel, Acousto-Optics (1988), and F. Marks, et al., xe2x80x9cA Comprehensive Approach to Breast Cancer Detection Using Light: Photon Localization by Ultrasound Modulation and Tissue Characterization by Spectral Discrimination,xe2x80x9d SPIE vol. 1888 (1993) pp. 500-510. Changes in the local optical properties of tissue can be measured by intersecting an acoustic field with the radiation field. Specific implementations can provide three dimensional (3-D) information.
A problem not addressed previously is that human skin has an index of refraction for non-ionizing radiation significantly different from that of air. In addition, human skin is not smooth on a microscopic scale and may also exhibit irregularities on a macroscopic scale. In cases where a transparent compression plate is not used to flatten the breast to be imaged at the entrance and/or exit points of the optical radiation beam, or when the transparent compression plate makes poor optical contact with the skin, then the incident radiation and the exit radiation will be partially reflected and experience additional scattering at the skin surface.
In addition, breasts are non-homogenous objects which lack uniform physical dimensions. The thickness of breast tissue over a region to be imaged may not be consistent. For a source with a limited coherence length (e.g., used in heterodyne detection or time-of-flight holography) or a pulsed source the optical flight time between source and detector (typically a fixed distance apart) depends not only on the types of tissue encountered as radiation passes through the breast, but also depends on the total thickness of tissue the light must traverse.
Prior devices and methods do not address these concerns.
The present invention comprises an apparatus and method directed to enhancing the image obtained from a high resolution breast imaging device utilizing non-ionizing radiation having a narrow spectral bandwidth. In addition, the present invention addresses the problems associated with the irregularities of human skin and the lack of physical uniformity in breasts. Notably, inefficient radiation coupling into and out of the breast and disparities in total radiation path lengths due to variations in total tissue thickness can be reduced by incorporating radiation coupling materials into the imaging system. Radiation coupling materials (also referred to as xe2x80x9cindex matchingxe2x80x9d materials), typically fluids or gels, can be used to improve transmission into and/or out of the breast as well as minimize disparities in the total radiation path length due to variations in total tissue thickness. One example of a possible index matching fluid is water.
As in applicants"" prior disclosures, the present invention utilizes a collimated light (radiation) source of narrow spectral bandwidth (such as generated by a laser, waveguide, phased array, etc.) to produce a beam or a number of beams of small spatial dimensions which, in turn, are used to obtain images of a breast with high spatial resolution. As is also described in applicants"" prior disclosures, the breast to be imaged is preferably compressed. However, the compression plates used to compress the breast need not be of the same size and one or both plates can be fixed or mobile. Greater compression (reduction in optical path length) is possible if a small area of the breast is compressed rather than compressing the entire breast at once (as in traditional x-ray mammography). It is possible to contour one or both plates in order to attain additional compression (and, therefore, a reduction in optical path length) beyond that expected from a reduction in plate size alone while reducing patient discomfort normally associated with breast compression. As is described above, a reduction in optical (radiation) path length by reducing the effective scatter volume aids in scatter reduction, improves image sensitivity, and reduces the power requirements of the optical source. These benefits apply to imaging techniques which use conventional optical collimation and/or time-of-flight methods (e.g., ballistic, snake-like, coherence, partial coherence, heterodyning, homodyning).
The present invention also relates to improving the radiation coupling with the skin surface of the subject breast. As is described above, where a transparent compression plate is not used to flatten the breast to be imaged at the entrance and/or exit points of the optical (radiation) beam, or when the transparent compression plate makes poor optical contact with the skin, then the incident radiation and the exit radiation will be partially reflected and experience additional scattering at the skin surface. Radiation which is scattered near the exit skin surface is more difficult to reject using coherence-based techniques such as time-of-flight. The preferred embodiment of the present invention uses an optical coupling material (for example, index matching liquids (such as water), gels, etc.) to reduce the optical index of refraction mismatch which occurs at a tissue-air and/or tissue-plate interface and, therefore, reduces radiation losses at the point of radiation entry and improves radiation transmission (reducing internal reflection) at the point of radiation exit. Optical coupling gels are widely used for optical fibers and other optical components. A commercial example of an optical coupling gel is Gel Code 0607 from Cargille Laboratories, Inc., 55 Commerce Road, Cedar Grove, N.J. 07009. Using an optical coupling material can be advantageous in transmission, backscatter, and optical computed tomography imaging. The coupling material can also aid in the dissipation of heat from the region being irradiated. Breast compression plates, as discussed above, can be constructed with materials that offer an index of refraction appropriate for optical coupling.
Furthermore, an optical coupling material (with appropriate index of refraction and/or scattering properties) can be used to minimize discrepancies in the path length differences due to non-uniform tissue thickness over the region of interest. This is particularly important for techniques which utilize coherence properties of the radiation field, such as pulsed radiation which is evaluated by utilizing time-of-flight analysis. Optical coupling materials can be chosen on the basis of their absorptive properties as well as their index of refraction and scattering characteristics. Such materials can provide scatter reduction by the additional attenuation of radiation which travels longer paths through the absorptive optical coupling material.
The present invention also relates to reducing the effects of patterned (structured) collimators on image quality by moving the patterned collimators in a reciprocating fashion.
The present invention also relates to acquiring additional information about tissue characteristics by intersecting an acoustic field with the radiation field.
The present invention also relates to estimating corrections for scatter by using two or more sources of radiation with distinguishable properties such as wavelength or polarization.
The present invention also relates to devices and techniques to overcome the limitations which substantial tissue thickness, irregular and rough surfaces, and variable (non-uniform) tissue thickness impose on optical and acousto-optical imaging. Consecutively compressing and then imaging small areas of the breast rather than compressing the entire breast at once results in significantly less patient discomfort, and consequently, increased compression, accuracy and image quality. This can be achieved with a variety of compression plate types wherein at least one of the plates is effectively smaller (provides a reduced contact area with the breast) than a typical compression plate used in x-ray mammography for compressing the entire breast at once. The area imaged is dependent on the desired tissue thickness (as well as the type of breast and the pain tolerance limit of the individual). In general, specialized compression plates as disclosed herein permit greater compression than is possible with compression plates designed to compress the entire breast. This reduction in the thickness of the tissue to be imaged is extremely important for optical and acousto-optical (and acoustic) imaging.
In addition, a coupling fluid or gel between the tissue surface (interface) and the compression plate may be introduced so as to minimize the effects of skin roughness and skin irregularities, and variable tissue thickness (in the nipple region and where tissue bulges). The skin surface is porous (rough) and it can have a number of irregularities (small indentations, creases, folds, etc.). Thus the skin surface can introduce angle-dependent scattering and transmission on the incident or exiting radiation which is unrelated to the composition and structure of the bulk breast tissue. This implies that the effects introduced due the presence of the skin surface will degrade the performance of any technique that is used to measure breast tissue properties. This includes methods such as time-of-flight (TOF) or other coherent imaging techniques, collimated beam imaging, diffusive imaging, etc. The coupling material can be transparent to optical and acoustic radiation or it can be tailored to have specific scattering and absorption properties. The coupling material not only reduces effects due to angle-dependent scattering and transmission at the entrance and exit skin surfaces, but it can also function as a lubricant (for image acquisition that involves moving a compression plate(s) along the surface of the breast). The coupling material can also conduct heat from the tissue surface.